Muscarinic m1, m3, and m5 receptors were shown to stimulate calcium influx when expressed in A9 and CHO fibroblasts. The calcium influx was blocked by cobalt suggesting that calcium crossed the membrane through a channel, but was not blocked by inhibitors of voltage sensitive channels. Muscarinic receptor-stimulated calcium influx was independent of IP3 and independent of the putative G protein binding domain of the muscarinic receptor. Signal transduction and pharmacology of the recently cloned cannabinoid, adenosine A1, and dopamine D2 receptors have been examined. The cannabinoid receptor inhibited adenylate cyclase activity in response to the addition of several classical and novel cannabinoid agonists. Receptors linked to adenylate cyclase inhibition through Gi, such as the adenosine A1, alpha-2 adrenergic, m2 and m4 muscarinic, and D2 receptors, augmented ATP-stimulated arachidonic acid release. CHO cells expressing the m5 muscarinic receptor were tumorigenic when injected into athymic nude mice and displayed anchorage independent growth into soft agar. When stimulated with the muscarinic agonist carbachol, there was a marked reduction in tumor formation and growth in soft agar. Muscarinic receptors linked to calcium mobilization may play a role in cell transformation.